The tubes in the primary loops of steam generators of nuclear reactors are subjected to high temperatures and pressures, radioactive bombardment, and attack by corrosive materials carried in the steam that passes through them. As a result, they frequently bend or deform from a straight or linear configuration. Because deformation usually precedes failure, it is necessary to periodically ascertain the deformation of these tubes so that dangerously deformed tubes can be plugged or replaced. In order to rationally decide how likely a deformed tube is to fail, one must know not only how much the tube is deformed, but also where along the tube the deformation occurs, as well as its direction.
Since the tubes are not visible or otherwise accessible from the outside, this information must be obtained by examining the inside of the tubes. The examination must be as precise as possible, of course, for otherwise tubes in acceptable condition will be unnecessarily plugged or replaced, or worse, a defective tube will not be detected and conceivably could fail, permitting contaminated water to enter the secondary loop. The problem is difficult because the tubes, of which there may be several thousands, are vertical, and can be only 1/4 to 1 inch in inside diameter, yet longer than 15 feet. In addition, the area where the examination must be performed is highly radioactive, so the examination must be done as quickly as possible to reduce radiation exposure to humans.